The objective of this research is to provide animal data that are relevant to questions concerning the risk of mammography. It is thought that little new data will be forthcoming from epidemiological studies of irradiated human females that pertain to the risks of mammography. We thus propose to study the rat model-system of radiation-induced mammary carcinogenesis. It is believed that this rat model-system is a good and proper model-system for the human female because both the female rat and the human female share a common scopal mechanism of radiation-induced mammary carcinogenesis. We have chosen to take advantage of the interrelationship between dose-response and the effects of dose rate rather than attempt a megarat experiment devoted to dose-response relationships. To a large degree the ultimate effect of lowering the dose or lowering the dose rate is the same, i.e., a linear, non-threshold curve. We will irradiate female, Sprague-Dawley rats with a single dose of 200 rads of 137Cs gamma radiation given in either 2, 100, 1000 or 10,000 minutes and study the incidence of rats with mammary adenocarcinomas. We thus propose to test the hypothesis that progressive lowering of the dose rate will not lower the incidence. If no repair is observed (i.e., incidence is not lowered), where the dose rate has been lowered by a factor of 5000, then it is unlikely that repair would occur at lower doses. We would conclude that the linear dose-response should be accepted and that the risk per rad is independent of dose or temporal pattern of dose. If on the other hand, the risk per rad decreases but approaches a lower and constant value asymptotically, then limited repair, as well as a linear relationship of lesser slope will have been demonstrated. If complete repair is observed, then a curvilinear dose-response curve is likely where the risk per rad does indeed depend upon the dose and the dose rate. These results should be of qualitative value in predicting the situation in irradiated human females.